Conventionally, there is a well-known start assister of a fuel injection pump comprising a start-advancing mechanism for advancing a fuel injection timing by closing a sub port in a plunger part of the fuel injection pump.
This conventional start assister has a drain passage formed in a housing to be connected to the sub port. The drain passage is opened and closed, i.e., the sub port is opened and closed by a peripheral portion of the piston, wherein the fuel injection timing is advanced by closing the sub port.
An actuator such as a temperature-sensing member, a solenoid, or the like is installed in the start assister. The sliding of the piston for opening and closing the sub port depends on expansion and contraction movement of a pin of the actuator.
In the housing are formed upper and lower chambers over and under the piston, and bored respective passages for connecting the upper and lower chambers to a fuel gallery.
With respect to the conventional start assister having a temperature-sensing member serving as the actuator, the pin of the temperature-sensing member is arranged in higher-pressured one of the upper and lower chambers over and under the piston. Therefore, the temperature-sensing member is subjected to change of pressure in the corresponding chamber and liable to be permeated by fuel oil in the corresponding chamber, thereby being possible to be damaged or deteriorated.
The temperature-sensing member, solenoid, or the like, serving as the actuator, is fixedly installed in the housing. The standardized start assister is unable to have the installed actuator replaced with another type actuator, thereby being difficult to correspond to various demands.
Moreover, even when the sub port is closed by the piston, fuel oil in a fuel oil chamber pressurized by a plunger leaks and flows into the upper and lower chambers over and under the piston from an inlay-gap between the housing and piston through the drain passage and the bores connecting the upper and lower chambers to the fuel gallery, so that the advance of fuel injection timing may be insufficient.
To prevent the fuel leak into the upper and lower chambers, the inlay-gap between the housing and piston requires complicated processing and management as precise as the inlay-gap between the plunger and the plunger barrel.